Eddy current testing is a non-destructive test technique for detecting defects or flaws in tubes and is based on inducing electrical currents in the material being inspected and observing the interaction between these currents and the material. The technique involves the use of a transmitter coil, through which a current flows, to induce a magnetically induced current (an eddy current) to be generated in the test sample. The flow of eddy currents is distorted in regions of defects or deformations. Such eddy currents, in turn, induce a current in a nearby receiver coil which is then used to determine the presence of defects in the tube. Since this technique is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the sample material being tested must be electrically conductive.
When inspecting for defects, maximum responses are achieved when the defects are perpendicular to the flow of eddy currents. If the eddy currents flow parallel to a defect, there will be little distortion of the eddy currents and a minimum response will be achieved; thus, it would be difficult to detect such defects. A conventional internal circumferential probe (i.e a bobbin probe) induces a flow of eddy currents parallel to the windings of a coil and, therefore, circumferential in direction. Thus, circumferential defects, those parallel to the path of such eddy currents, will not be sensed. Therefore, the orientation of a defect with respect to the coils of the probe affects the degree of sensitivity of the probe to the defect.
The detection of circumferential cracks is one of the most difficult eddy current inspection problems. Conventional eddy current techniques have low sensitivity to circumferential cracks and cannot be used to reliably estimate crack depth. It is a recognized problem that reliable detection and sizing of circumferential cracks, fretting wear, shallow internal defects etc. is made more difficult by the fact that they frequently occur in defect prone regions such as under tube-sheets or support plates and in transition regions of finned tubes. In such cases, the structural features surrounding the tube under inspection introduce marked deviations in output signals, thus making detection of defects very difficult if not impossible. Further, circumferential cracks normally occur in defect prone regions such as under support plates or at U-bends, where the tubes are often deformed, thereby making inspection difficult. Utilities around the world have experienced circumferential cracks in steam generator tubing; it is a major inspection problem.
Various probes have been proposed for inspecting cylindrical or tubular components. For example, probes of this nature have been described in the following United States Patents:
U.S. Pat. No. 3,952,315 (Apr. 20, 1976; Cecco et al) PA1 U.S. Pat. No. 4,808,924 (Feb. 28, 1989; Cecco et al) PA1 U.S. Pat. No. 4,808,927 (Feb. 28, 1989; Cecco et al)
The specifications of these prior art patents are incorporated herein by reference.
The probe disclosed in U.S. Pat. No. 4,808,927 comprises a bobbin type transmitter coil associated with pancake type receiver coils. Accordingly, the eddy currents generated by the probe flow circumferentially and, therefore, such probe is not capable of detecting circumferential defects.
The probe disclosed in U.S. Pat. No. 4,808,924 makes use of bracelets of multiple pancake type transmitter and receiver coils for detecting localized defects, including circumferential cracks, in a tube. This probe detects any defects including defects under support plates as a result of the "circumferential compensation" achieved by the orientation of the probe coils. In other words, concentric changes or gradual circumferential variations are rendered invisible (or compensated) in the output. Further, the bracelets of coils are rotated about the central axis of the tube so as to provide 100% coverage of the tube surface. In addition, the coils of the '924 probe are arranged so as to render primarily an absolute output signal. However, such absolute output of the probe makes it difficult to detect small cracks of any orientation in regions containing tube deformations or in the presence of deposits (for example copper). Further, it has now been found that some of these difficulties are the result of interference caused by the effect of one transmitter coil on adjacent receiver coils at different distances. This finding has led to the determination that the amplitude and phase of the output signal of a receiver coil is a function of the square of the distance between such coil and a transmitter coil. Accordingly, if the current in a receiver coil is generated by various transmitter coils at different distances, the resulting output cannot be analyzed accurately. In addition, the compensating design of the probe makes analysis of the output difficult.